Today, the need for correctly verifying an individual's identity has become essential both to prevent identification (ID)-based fraud in our new age of electronic information and payments as well as to allow or prevent physical and/or electronic access in numerous situations to ensure and maintain the integrity of premises and/or systems. For example, “ID-based fraud” includes such items as improper payment card usage, theft and misappropriation of identity information and even the use of false credentials. Examples of “access” include access to specific information, services, and/or communication facilities, use of specific devices or equipment such as computers, mobile phones, handheld devices, automobiles, or machinery, physical access to a restricted area or premises, or even entry into a country as with a passport. Biometric information or signatures, such as a fingerprint pattern, a voiceprint pattern, physical appearance (a face, an ear, an iris, a retina, and the like, of a person), biological assay, and the like, can be used as a more reliable, secure, and convenient method for verifying a person's identity compared with the traditional method of password-based verification.
Among biometric signatures, fingerprints have been traditionally one of the most frequently used and various types of fingerprint sensors have been developed. For example, a capacitive fingerprint sensor typically measures the electrical capacitance between a number of sensing elements arranged in an array and the fingerprint contour, i.e., ridges and valleys of the finger skin. However, such capacitive fingerprint sensors are typically affected by the condition of a finger, such as moisture and/or dirt on the finger. An optical, contactless-type fingerprint sensor is neither affected by finger condition (dry/humid) or smearing effect. However, optical fingerprint sensors typically require complex detection circuitry and extensive data processing for pattern recognition, they are not suitable for small portable devices with limited computational capacity, or “card” applications such as smart cards, credit cards, bank cards, driver's licenses, passports, and the like. In addition, the biometric sensor needs to be sufficiently small in size (i.e., thin and light), durable, and also cost effective in order to be implemented in such card applications. For example, the cost of conventional fingerprint sensors including a sensor array using standard CMOS technology would make it economically impractical at the present time to be implemented in such card applications.